1. Field of the Invention
The present invention relates to a driving device and optical instrument. More particularly, the present invention relates to a driving device and optical instrument in which a piezoelectric element is used to apply force to a device to be driven.
2. Description Related to the Prior Art
An optical instrument such as a camera includes an actuator as a driving device, and a mechanical element for being driven by the actuator. For example, U.S. Pat. No. 5,225,941 (corresponding to JP-A 4-069070) and U.S. Pat. No. 5,589,723 (corresponding to JP-A 7-274543) disclose a use of a piezoelectric actuator in a camera as an electromechanical converting element or transducer.
In FIG. 21, a piezoelectric actuator of the prior art for use with a lens is illustrated. A lens barrel 100 as an element to be driven includes a projection 101 and holes 101a and 101b. The projection 101 projects from the periphery of the lens barrel 100. The holes 101a and 101b are formed to extend in the optical axis direction. A drive shaft 102 extends through the holes 101a and 101b, and is frictionally engaged in a slidable manner. Holes 104 and 105 are formed in a frame portion 103. The drive shaft 102 is inserted through the holes 104 and 105 and supported in a slidable manner in the axial direction. A piezoelectric element 106 in a piezoelectric actuator has a first end secured to the frame portion 103, and a second end secured to an end of the drive shaft 102.
The piezoelectric element 106 response to a drive pulse as an electric signal, and either expands or contracts in its direction of thickness, to shift the drive shaft 102 in the axial direction. The drive pulse sent to the piezoelectric element 106 has a waveform defined by a combination of a period of a slow rise of the voltage and a succeeding period of a quick drop of the voltage. The piezoelectric element 106 shifts by the expansion at a low speed in the direction C in the period of the slow rise, and shifts by the contraction at a high speed in the direction C in the period of the quick drop.
In the period of the slow rise of the voltage, the drive shaft 102 shifts in the direction C at a low speed. The lens barrel 100 shifts together with the drive shaft 102 in the direction C in keeping frictional coupling with the drive shaft 102 at the holes 110a and 101b. In the succeeding period of the quick drop of the voltage, the drive shaft 102 shifts at a high speed in reverse to the direction C. The holes 110a and 101b are released from the frictional coupling with the drive shaft 102. The lens barrel 100 is kept positioned by the inertia. As a result, a relative position of the lens barrel 100 relative to the drive shaft 102 is changed. The lens barrel 100 is moved in the direction C from the initial position.
The drive pulse of this form is sent to the piezoelectric element 106 consecutively, to move the lens barrel 100 in the direction C continuously. If movement of the lens barrel 100 is desired in the direction opposite to the direction C, a drive pulse is sent to the piezoelectric element 106, the pulse having a waveform defined by a combination of a period of a quick rise of the voltage and a succeeding period of a slow drop of the voltage.
There are problems in the above-indicated prior documents. A base end of the drive shaft 102 is secured to the piezoelectric element 106. A distal end of the drive shaft 102 is loosely connected with the frame portion 103 at the hole 104. Driving force of the piezoelectric element 106 is difficult to transmit in a reliable manner at the distal end of the drive shaft 102. An amount of movement of the lens barrel 100 per one pulse decreases gradually in a direction from the base end to the distal end of the drive shaft 102 in the combined use of the lens barrel 100 and the drive shaft 102. It is impossible to control the lens barrel 100 stably in the vicinity of the distal end of the drive shaft 102. The same shortcoming remains if a device other than 100 is connected for being driven.